The conventional Michelson interferometer, which essentially consists of a beam splitter and a first and a second mirror, is an example of an interferometer of the above type. In the Michelson interferometer, the beam splitter reflects part of the incident beam towards the first mirror, and transmits part of the incident beam towards the second mirror. The two mirrors reflect the beams back to the beam splitter, which combines them to a single beam which then impinges upon a detector. Depending on the difference in distance between the beam splitter and the respective mirrors, a constructive or destructive interference arises between the beams. This interference can be registered with the aid cf the detector.
A Michelson-type interferometer may, for instance, be utilised in a Fourier transform spectrometer for determining the spectrum of the light from a broadband light source. In the conventional Fourier transform spectrometer, the one mirror serves as scanning mirror and is linearly displaced so that the distance to the beam splitter, and hence the interference pattern on the detector, is continuously changed. In this way, an interferogram can be registered by the detector. With the aid of Fourier transformation, the spectrum of the incident light can then be determined.
An inconvenience of conventional interferometers is that the optical components have to be accurately set in relation to each other in order to obtain an interference pattern fit for use.
In the Michelson interferometer, the beam splitter is so arranged that the angle of incidence of the incident beam is 45.degree.. Furthermore, the first and the second mirror are orthogonal, and the first and the second beam impinge upon the mirrors in the normal direction. If, say, the one mirror is so displaced that the beam does not arrive in the normal direction, the fringe density of the interference pattern will increase. When there is a considerable displacement, the fringes cannot be observed as a result of too high a density.
In the Fourier transform spectrometer, the accurate setting has to be maintained during the whole linear displacement of the scanning mirror, which makes the mounting critical.
In order to lower the high requirements for an accurate setting during the displacement, the mirrors may in known manner be replaced with retroreflectors. However, retroreflectors are expensive, and it is furthermore impossible to compensate for any angle errors that may arise in production or as a result of temperature changes, ageing and the like.
In order to maintain an accurate setting of the optical components, it is further known to utilise fixed mirrors for reflecting the two beams back to the beam splitter and to alter the path length of the one beam with the aid of a rotatable element, for instance a rotatable etalon. EP 0 491 435 discloses an interferometer having two parallel, opposed mirrors, which are rotated in order to alter the path length to the one fixed mirror. However, this technique has the disadvantage of the beams moving across the surfaces of the fixed mirrors, such that extremely high requirements are placed on the quality thereof. Furthermore, the path length can only be altered to a minor extent.
In the literature, there are examples of further developments of the above principles intended to enable an increase of the difference in path length between the two beams. In EP 0 314 103 and U.S. Pat. No. 5,066,990, for instance, the rotating element consists of a double pendulum having a retroflector on each arm. By swinging the double pendulum, the path length of the one beam is reduced, whereas the path length of the other beam is increased. However, the inconvenience associated with the retroreflectors remains.
In U.S. Pat. No. 5,159,405 and U.S. Pat. No. 5,150,172, the rotating element consists of two parallel mirrors, between which the two beams are reflected a number of times. When the two mirrors are turned, the path length of the one beam is lengthened, whereas the path length of the other beam is shortened. However, the inconvenience of the beams moving across the mirrors still remains.